1. Field of the Invention
This invention relates to a vinyl chloride resin composition and a medical instrument made of the resin composition. More particularly, this invention relates to a vinyl chloride resin composition which enjoys high safety and exhibits high stability to withstand heat, light, and radiation and to a medical instrument made of the resin composition and enabled to be sterilized safely with radioactive ray.
2. Description of the Prior Art
Generally, vinyl chloride resin excels in workability, physiological safety, and transparency and features low price and, therefore, finds widespread utility as a resin for the production of various medical instruments such as bags for blood, bags for transfusion fluids, and conduits for blood transfer.
The vinyl chloride resin, however, suffers from the serious drawback of succumbing to thermal decomposition. By the action of heat, the vinyl chloride resin is caused to undergo decomposition due to removal of hydrochloric acid by some mechanism yet to be elucidated. In the air, it is oxidized and caused to undergo molecular cleavage and cross-linking bonding at a sacrifice of physical properties including freedom from brittleness.
The tendency of the vinyl chloride resin for degradation by heat naturally entails various problems.
When a vinyl chloride resin piece is fabricated in a given shape, especially when the vinyl chloride resin is of rigid grade or of high polymerization degree, since it has high melt viscosity and generates heat of friction heavily, the resin temperature rises sharply and, consequently, undergoes accelerated thermal decomposition and, during the course of fabrication, suffers from thermal deterioration within the molding machine. When the resin happens to undergo a lengthy residence within the molding machine, the such resin manifests itself as a black extraneous portion in the shaped article obtained by the fabrication. When the vinyl chloride resin is exposed to high shearing strength during the course of fabrication as in injection molding, the resin under treatment is tarnished in part to impair the outward appearance of the shaped article resulting from the fabrication. Thus, the shaped article is rejectable as an item of commerce. Moreover, there is the possibility that the interior of the molding machine must be cleaned frequently for removal of the stagnant resin.
Further, the vinyl chloride resin possesses the serious drawback of being degraded by light and radiation. To be more specific, by the action of heat or radiation of wavelength of not more than 3,000 A, it undergoes decomposition due to removal of hydrochloric acid by some mechanism yet to be elucidated. As the decomposition starts, it accelerates removal of hydrochloric acid in the manner of chain reaction and brings about sequential formation of a conjugate double bond otherwise called a polyene structure (zipper reaction). It is generally held that when 8 or more double bonds form a chain within the conjugate polyene, they form a chromophore within the visible zone [Hirayama: Journal of Japan Chemical Society, 75, 27, 667 (1954)] and cause coloration of the resin. As this reaction proceeds, the resin grows black and entails molecular cleavage and cross-linking bonding and can no longer withstand impacts inflicted during daily use.
This tendency of the vinyl chloride resin toward degradation by the action of heat, light, and radiation naturally entails various problems detrimental to the use of this resin in apparatus for medical treatment.
For example, as a flexible vinyl chloride resin composition for use in a medical instrument, the vinyl chloride composition which comprises 100 parts by weight of polyvinyl chloride, 30 to 80 parts by weight of di-2-ethylhexyl phthalate, 0.01 to 5 parts by weight of a calcium-zinc type stabilizer, 0 to about 10 parts by weight of epoxidized soybean oil as a stabilizing aid, and optionally 0 to about 5 parts by weight of lubricant, etc, has been provided to date.
When any of these medical instrument is sterilized by means of radiation, the vinyl chloride resin is deteriorated by exposure to radiation and, consequently, is discolored with passage of time to the point of becoming unsuitable for use as a medical instrument.
Unlike the method of thermal sterilization which requires the apparatus to possess high resistance to heat and yield minimally to heat distortion or unlike the method of sterilization by the use of a bactericide such as ethylene oxide gas which requires the treated apparatus to be left standing long before it becomes fit for safe handling, the method of sterilization by radiation is excellent in the sense that the sterilization can be carried out easily at low temperature. Despite the characteristic feature, this method is not safely applicable to the vinyl chloride resin.
Numerous studies have been made to date for improving to a sufficient extent the stability of the vinyl chloride resin to withstand heat, light, and radiation. One such study has led to acquisition of the knowledge that an organic compound of such heavy metal as lead or cadmium is highly effective in stabilizing the vinyl chloride resin. Since the organic compound of such a heavy metal as described above is highly toxic and, therefore, harmful to man, it cannot be incorporated in the material for apparatus for medical treatment. The vinyl chloride resin composition which is now accepted generally for use in medical instrument incorporates a metallic soap tye stabilizer of calcium or zinc which is held not to be toxic. The stability improved by the use of such a stabilizer as described above is not sufficient.
As an organic stabilizer there is a report [W. Szymanski, G. Smietanska, journal of Applied Polymer Science, 19, 1955 (1975)] and a .beta.-aminocrotonic ester is effective in improving the stability of the resin to withstand radiation and in repressing the removal of hydrochloric acid. The medical instrument made of vinyl chloride resin incorporating the .beta.-aminocrotonic ester as a stabilizer, on being sterilized with radiation, is discolored to a greater extent than the apparatus not containing this organic stabilizer in the resin. Thus, this apparatus fails to satisfy the standard of safety and does not fit medical use. In the case of a medical instrument made of vinyl chloride resin incorporating .beta.-diketone (product of Rhone Poulenc marketed under trademark designation of Carrens DK-1), a substance claimed to be a highly effective color inhibitor, when this apparatus is sterilized by radiation, it is discolored to the same extent as or to a greater extent than the apparatus made of vinyl chloride resin not incorporating this color inhibitor. Thus, this apparatus also does not fit applications requiring sterilization by radiation.
An object of this invention, therefore, is to provide an improved vinyl chloride resin composition and a medical instrument which is made of the vinyl chloride resin composition.
Another object of this invention is to provide a vinyl chloride resin composition enjoying high safety and exhibiting high stability to withstand heat, light, and radiation and a medical instrument which is made of the resin composition.
Yet another object of this invention is to provide a vinyl chloride resin composition which exhibits improved dynamic thermal stability during the course of fabrication.
A further object of this invention is to provide a medical instrument which can be safely and effectively sterilized by radiation.